Get tests passing in Docker env

[rebeccacremona]

• installs node and npm packages
• installs a local redis on the web/worker image for use by pytest-redis, which cannot be configured to use a remote redis at this time.....
• uses named volumes and the :delegated to smooth heavy I/O. Might not be necessary for the local redis, but so far as I can tell harmless
• does not ADD code to the image, since up-to-date code is supplied via mounted volumes

Note that tests pass even when the worker containers crash. (Perma doesn't test it's celery workers either)

[bensteinberg]

looks great, thanks!